feat: add Server header with version

Ref: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server
Fixes: #367

pkg/internal/itest/http_fetch_test.go

@@ -1068,6 +1068,7 @@ func TestHttpFetch(t *testing.T) {
 						req.NoError(err)
 						req.Failf("200 response code not received", "got code: %d, body: %s", resp.StatusCode, string(body))
 					}
+					req.Regexp(`^lassie/v\d+\.\d+\.\d+-\w+$`, resp.Header.Get("Server"))
 					req.Equal(fmt.Sprintf(`attachment; filename="%s.car"`, srcData[i].Root.String()), resp.Header.Get("Content-Disposition"))
 					req.Equal("none", resp.Header.Get("Accept-Ranges"))
 					req.Equal("public, max-age=29030400, immutable", resp.Header.Get("Cache-Control"))

pkg/server/http/ipfs.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"strconv"
 
+	"github.com/filecoin-project/lassie/pkg/build"
 	"github.com/filecoin-project/lassie/pkg/retriever"
 	"github.com/filecoin-project/lassie/pkg/storage"
 	"github.com/filecoin-project/lassie/pkg/types"
@@ -147,6 +148,7 @@ func ipfsHandler(fetcher types.Fetcher, cfg HttpServerConfig) func(http.Response
 
 		carWriter.OnPut(func(int) {
 			// called once we start writing blocks into the CAR (on the first Put())
+			res.Header().Set("Server", build.UserAgent) // "lassie/vx.y.z-<git commit hash>"
 			res.Header().Set("Content-Disposition", fmt.Sprintf("attachment; filename=%q", fileName))
 			res.Header().Set("Accept-Ranges", ResponseAcceptRangesHeader)
 			res.Header().Set("Cache-Control", ResponseCacheControlHeader)